Repeaters are widely used in wireless communication systems in order to re-amplify electromagnetic signals. Repeaters receive, amplify and retransmit signals of a particular channel. Due to the amplification, the signal can propagate over longer distances and thereby increase the reach of the signal, or, a better signal quality is provided at a receiver of the signal, i.e., a better signal to noise and interference ratio is provided. Repeaters may also be referred to as layer-1 relays or amplify-and-forward relays.
A signal, which has been amplified in a repeater, can either be transmitted by the repeater on a different frequency band than the one the original signal was received in, which is referred to as “frequency translating”, or the amplified signal can be transmitted on the same frequency band as the original signal was received in, which is referred to as “on-frequency”. In the latter case, the transmitted amplified signal will cause self interference, since it will interfere with the reception of the original signal in the repeater. This self-interference can be avoided, e.g. by means of separated antennas, self-interference cancellation techniques, or by electronic components, such as circulators.
A conventional repeater continuously amplifies the entire channel bandwidth. The repeater amplifies all resources such as TDMA (Time Division Multiple Access) time slots, FDMA (Frequency Division Multiple Access) sub channels, or OFDMA (Orthogonal Frequency Division Multiple Access) resource blocks, even when the resources are currently not used for transmission, or when the resources do not need repeater support in order to reach their destination. Thus, energy is wasted whenever the repeater amplifies these resources in the described situations.
Conventional repeaters are used in order to amplify resources comprising desired downlink carrier signals or desired uplink carrier signals, which signals need repeater support. However, the repeaters also amplify resources with undesired signals. Undesired signals on these resources are for instance, apart from self interference:                Interference from a neighboring cell. If this interfering signal is amplified by the repeater it will degrade the signal quality within the cell;        Signals which do not need repeater support in order to reach their destination. These signals cause interference to neighboring cells if they are amplified, and may also interfere with the original direct signal in a receiver if a processing delay introduced by an on-frequency repeater exceeds the allowed limit;        The receiver noise of the repeater, which causes interference within the cell and towards neighboring cells if it is amplified.        
In a conventional repeater, the transmit power of the repeater is equally distributed across the amplified channel bandwidth. The power amplifier of the repeater constitutes the limit to the power spectral density of the amplified signal. If the transmit power of the repeater were to be concentrated only to certain frequency resources, e.g. subcarriers in OFDMA or sub channels in FDMA, the power spectral density of the amplified signal could be increased. This would either enable the use of less powerful and less expensive amplifiers in the repeater or it would enable an increase of the signal quality and range.
One way to alleviate the above mentioned problems is to design a frequency selective repeater, i.e. a repeater which only amplifies the resources, e.g. resource blocks in LTE, which are beneficial for a communication. A frequency selective repeater can be controlled by a base station to only repeat resources that are in use by mobile stations, which are scheduled by that base station. Furthermore, a mobile station with a strong radio link to the serving base station does not need support from the repeater and hence resources used to communicate with that mobile station should not be amplified by the repeater, in order to avoid that unnecessary interference is forwarded by the repeater.
However, there are a number of reasons why frequency selective repetition as described above does not work directly in LTE downlink, which is also illustrated in FIGS. 4 and 5:                The downlink control channels cover the whole bandwidth, which means that the whole bandwidth must be repeated;        The downlink demodulation reference symbols are used also for mobility measurements, which means that all resources carrying demodulation reference symbols must be repeated;        Paging messages could be scheduled anywhere, which means that paging messages could be lost if certain resources were not repeated;        Mobile terminals, also called UEs (User Equipment), need to be able to receive the BCH (broadcast channel) and the PSS and SSS (primary and secondary synchronisation signals), which means that the resources used for BCH, SSS, and PSS must be repeated.        
Further, frequency selective repetition in the LTE uplink is complicated by the following, which is illustrated in FIG. 6:                Mobile terminals could be scheduled anywhere in the frequency domain, which means that data related to a mobile terminal may be lost if the resources used by that mobile terminal are not repeated.        The control channels are located at the frequency edges, which makes filtering challenging.        The RACH (Random Access Channel) must reach the base station, which means that the resources used for RACH must be repeated.        
E-UTRAN, which is also denoted Long-Term Evolution, LTE, of UTRAN, as standardised in Release-8 by the 3GPP, supports bandwidths up to 20 MHz. However, in order to meet the upcoming IMT-Advanced requirements (International Mobile Telecommunications), work on advanced systems has been initiated, which may be referred to as “LTE-Advanced”. One of the parts of LTE-Advanced is to support bandwidths larger than 20 MHz. One important requirement on developers of LTE-Advanced is to assure backward compatibility with LTE Release-8 terminals, i.e. that legacy terminals should be able to function within LTE-Advanced. This should also include spectrum compatibility. That would imply that an LTE-Advanced carrier, wider than 20 MHz, should have the possibility to appear as a number of LTE carriers to an LTE Release-8 terminal. Each such carrier can be referred to as a Component Carrier. The straightforward way to obtain such LTE-Advanced carriers wider than 20 MHz would be by means of carrier aggregation. Carrier aggregation implies that an LTE-Advanced terminal can receive multiple component carriers, where the component carriers have, or at least have the possibility to have, the same structure as a Release-8 carrier. Carrier aggregation is illustrated in FIG. 1, where five component carriers of 20 MHz each are aggregated to a 100 MHz carrier.